In the air conditioning industry, heat pump systems are used for both cooling and heating. When the system is heating, the outside evaporator becomes frosted and requires periodic defrosting. The most popular defrosting system includes a timer operating switches in sequence which puts the system through a fixed defrost cycle at fixed intervals. The frequency of defrosting depends on the location of the system. For example a system operating in a humid location such as Florida requires more frequent defrost cycles than the same system operating in a dry location such as Arizona. In the past it has been the practice of manufacturers to purchase their timers in an assortment of different timings between defrost cycles, and ship units with the proper timings to the different sections of the country. This involves extra inventory, extra costs in manufacturing and shipping, and the ever present danger of shipping the wrong unit to a given section of the country. Prior to applicant's invention it was not economic to provide all units with an identical adjustable timer as the extra cost of the adjustable feature was prohibitive.